In air conditioning a space for temperature, especially when cooling is required, it is desirable to deliver air at a temperature that is not uncomfortable to occupants of the space who happen to be in the path of the delivered air. On the other hand, it is desirable to provide air to an extreme temperature in order to limit the size of supply ducts and other equipment. Induction mixing boxes have been employed to accomplish both of these desirable results. Primary air at a relatively low constant temperature is carried through small ducts to an induction mixing box, in which flow of the primary air is employed to induce flow of secondary air thereinto. The secondary air is usually return air from the space, so that its temperature is probably at approximately the sensed space temperature. By properly proportioning the flows of primary and secondary air into the mixing box, the resulting mixed air has a temperature below the desired space temperature, but it is not uncomfortable to those occupants of the space who are in its path. Since it is the primary air that provides the required cooling, it is the volume rate of flow of primary air that must be controlled in order to maintain the conditioned space at substantially the desired set point temperature. By controlling the volume rate of flow of secondary air inversely as the primary rate, the volume rate of flow of mixed air into the controlled space is maintained substantially constant, so that air circulation in the space remains substantially unchanged regardless of the cooling requirements. U.S. Pat. Nos. such as Kennedy 3,114,505, issued on Dec. 17, 1963; Schach Re. 26,690 of 3,361,157, issued on Jan. 2, 1968; and Zille and Engelke 3,583,477, issued on June 8, 1971 are representative of the development of such induction mixing boxes. In each of these patents, one damper is employed to maintain a constant static air pressure upstream from a primary flow control damper, which then provides a desired volume rate of flow of primary air thereby controlling the amount of cooling supplied, while a secondary air damper determines the volume rate of flow of secondary air in order to maintain a substantially constant flow of mixed air into the controlled space.
A U.S. Pat. No. 3,809,314, issued on May 7, 1974 to Engelke and Zille, discloses a resettable constant volume air damper control.